You can find the Silent Library on the lowest floor of the Gauntlet of Shar, along the corridors from the third Faith-Leap trial room. It might look like there's a barrier over the door, but it's just that big Librarian orb enemy in the centre of the room shh-ing everyone, which you should destroy right away if you want to remove silence and cast spells again. You need to do the puzzle to get the Spear of Night for Shadowheart.
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